Thanks largely to the remnants of Hurricane Gustav, rain soaked much of the central Midwest the first nine days of September. However, the storm track left the northern Midwest, particularly Minnesota, and the far southeastern portions of Kentucky untouched. Rainfall the first nine days of the month was 3 to 5 times normal from Missouri northeastward through Michigan, and in far northwestern Minnesota (Figure 1). In contrast, rainfall was less than 50 percent of normal from southeastern Minnesota into western Wisconsin, and 35 to 50 percent of normal in the eastern two-thirds of Kentucky. As a result of the continued dry weather, the September 9 U.S. Drought Monitor depicted an expansion of Moderate (D1) drought in the Midwest, and a small area of Severe (D2) drought is now depicted in north-central Minnesota (Figure 2).

There was a 12°F difference in temperature departures across the Midwest this period. Average daily temperatures ranged from 8°F to 10°F below normal in the far western portions of the region to more than 2°F above normal in much of Kentucky and Ohio (Figure 3). The warmer portions of the Midwest were areas generally south of the path of Gustav's track that remained in strong southerly flow until late in the period. Temperatures this period peaked on September 2, the day before Gustav entered the Midwest. A broad area of 90°F plus maximum temperatures extended from Kentucky northward through eastern Wisconsin and lower Michigan (Figure 4).

Gustav Takes Aim on Midwest

After slamming into the Louisiana Gulf Coast on Labor Day (September 1), Gustav rapidly weakened to Tropical Storm and then Tropical Depression strength as it pushed to extreme southwestern Arkansas and temporarily stalled (Figure 5). Meanwhile, a cold front had pushed through the northwestern half of the region (Figure 6). Soaking rains spread across Missouri on September 3, causing extensive flash flooding as 24 hour totals exceeded three inches across much of the state (Figure 7). By the morning of September 4 the remnants of Gustav had merged with the cold front and was now a wave of low pressure on the front (Figure 8). Heavy rain spread through Illinois and into Michigan with three to four inch amounts common, but the rain was confined to a relatively narrow band as the low raced northeast along the front (Figure 9). Some showers and thunderstorms continued along the front on September 5 in Ohio south into Kentucky, but the main energy associated with the remnants of Gustav had moved northeast into Canada and rain there was not particularly heavy or widespread.

Severe weather in the Midwest associated with Gustav was isolated. A weak tornado briefly touched down near Eagle Rock, MO (Barry County) on September 2, and an EF1 tornado was reported near Wheatfield, IN (Jasper County) on September 4.

A Cool and Wet End

A strong cold front moved through the Midwest on September 8, accompanied by a band of heavy rain from extending from northern Missouri through northeastern Illinois and into southern Michigan (Figure 10). The front also touched off a few severe thunderstorms in eastern Missouri, southern Illinois, and central Indiana. Locally heavy rain fell in southern Indiana and central Ohio. As high pressure settled over the Midwest on the morning of September 9, low temperatures dropped into the low and mid 30s across Minnesota and western Wisconsin, and some scattered frost was reported (Figure 11). Lows in the upper 30s were observed as far south as central Iowa.